Cellulose is the main component of higher plant cell walls and one of the most abundant organic compounds on earth. Wood contains approximately 50% cellulose, 30% hemicellulose, and 20% lignin. In the pulping process cellulose is separated from the lignin and hemicellulose in a fibrous form that is purified, dried, and shipped in large rolls. Cellulose has been used for thousands of years but its chemistry like all other biopolymers was discovered and explored with the beginning of the last century. Today extracted and purified cellulose and its derivatives are widely used in several different industrial applications such as textile, paper, paints and coatings, foodstuff, pharmaceuticals and the oil industry.
In the last three decades, defibrillation of cellulose fiber into micro- or nano-fibers with a diameter of less than 1 μm using high shear methods such as high pressure homogenization and other methods has attracted a lot of interest. These fibers are known as MicroFibrillar Cellulose (MFC). Cellulose defibrillation can be performed by a variety of methods as known from the literature. For example it could be performed by applying pure mechanical shearing of any cellulosic raw materials such as bleached & unbleached pulps, vegetables and fruits, wheat and rice straw, hemp and flax, bamboo, beet and sugar cane or ramie and cotton. It is known that chemical or enzymatic treatment of the cellulose raw material prior to the mechanical treatment greatly reduces the energy consumption during the defibrillation process.
A method for enzyme-assisted preparation of MFC nanofibers was presented by Henriksson (“An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers”, Henriksson et al, European polymer journal (2007), 43: 3434-3441). In 2006 Saito2 et al reported on the use of TEMPO-Catalyzed Oxidation of Native Cellulose to produce Microfibrils (“Homogeneous Suspensions of Individualized Microfibrils from TEMPO-Catalyzed Oxidation of Native Cellulose”, Saito, Biomacromolecules (2006), 1687-1691).
In the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) process a free radical is used to oxidize the primary hydroxyl group in position 6 in the cellulose structure and convert it into carboxyl group that provide repulsive forces between the fibril's. In this process the bonding between the fibrils (such as van der Waals' forces and hydrogen bonding) are disrupted and weakened and this promotes the defibrillation process. The separated primary microfiber has a diameter in range of 5-100 nm and a length that can be varied within the range of 1-100 μm. The diameter of the fibril's can be controlled by using the desired energy input as well by adjusting the treatment condition prior the defibrillation however the length of the fiber is more difficult to control. The size of the defibrillated cellulose fiber depends on the treatment condition.
Another chemical treatment prior to the defibrillation can be the carboxymethylation of cellulose fiber to produce carboxymethylated MFC (CM-MFC) (“The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes”, L. Wågberg et al., Langmuir (2008) 24(3), 784-795,).
Both carboxymethylation and TEMPO treatment introduce anionic changes on the surface of the fibrils additional to the hydroxyl groups that already exist on the cellulose fibrils.
The MFC can be cationically charged if any cationic additives such as cationic surfactants or polymer or inorganic salts are added during the defibrillation process.
Such microfine fibrils with a high aspect ratio have extraordinary rheological and mechanical properties and a lot of research is being conducted to develop applications for such materials.
Bacterial MicroFibrillar Cellulose is produced by various species of Acetobacter organisms. The synthesis of bacterially produced cellulose (BPC) can be described as bottom-up synthesis where organisms build new polymeric materials (cellulose fibers) from monomeric spices (glucose units). BPC has, to a certain degree, a similar morphology to the defibrillated cellulose fiber (MFC) discussed above in terms of fiber morphology and size but it differs in the purity and the crystallinity. BPC has attracted a lot of attention in the last two decades due to its remarkable properties that can be used in many applications such as biomedical applications, papermaking, nanocomposites, electronic and acoustic devices, and foodstuffs. However, BPC is not commercially available in large quantities at a reasonable price due to the production complexity, but it has been used in small quantities in some applications. U.S. Pat. No. 5,350,528 discloses the use of BPC within a fracturing fluid composed of bacterial cellulose and a crosslinking agent.
To enhance the productivity of oil and gas wells, stimulation methods such as hydraulic fracturing or acidizing are well-known practice. Hydraulic fracturing fluids comprise mainly water as the fluidic phase, a proppant such as sand or ceramic materials with a defined size and strength to keep the fracture open, a viscosifier to carry and place the proppant into the formation and other chemicals that provide corrosion inhibition, fluid loss control, shale stabilization, etc. The commonly used viscosifiers in fracturing fluids are guar gum and its chemically modified forms such as hydroxypropyl guar (HPG), viscoelastic surfactant, and cellulose derivatives.
Normally guar gum, and its derivatives used in stimulation (hydraulic fracturing) fluids are crosslinked in order to reduce the amount of polymer that is pumped into the formation to minimize the potential of the formation damage due to the blockage of the pore throats by the polymers. It has also been found that crosslinking improves the thermal stability of polymers.
Crosslinking in this context is a reaction involving sites or groups on existing macromolecules or an interaction between existing macromolecules that results in the formation of a small region in a macromolecule from which at least four chains emanate. There are two main mechanisms of crosslinking by means of;    1) Physical crosslinking using ionic or electrostatic interaction. This is used to associate or crosslink the macromolecules. Hydrophobic interaction is also used to associate or crosslink macromolecules in aqueous solution to increase the rheology. Metal cations such as boric acid, or salts of aluminum, titanium or zirconium, or any organic positively charged molecules are used to create an interaction between the biopolymer chains. Such a crosslink is typically weak in nature and may be desirable in some applications where it is necessary for such bonds to be easy to break.    2) Chemical crosslinking where a covalent bond is created between the polymer chains. Polymerization reactions such as free radical or condensation polymerizations are used to chemically crosslink macromolecules such as biopolymers. Also difunctional molecules such as difunctional aldehydes (e.g. glutaraldehyde), or dichloroacetic acid that are able to react with the macromolecules are also used as crosslinking agent. Such crosslinking is hard to break and need chemical treatment to break them like the use of free radicals such as peroxide salts or hypochlorite, chlorate or bromate salts. Such crosslink may be desired in some oil well application such as water shut-off or enhanced oil recovery. Examples of such chemical crosslinkers for cellulose are formaldehyde and difunctional aldehydes (for example glutaraldehyde, dichloroacetic acid, polyepoxides, and urea). Some other crosslinking agents used for starch polymer that can be used with MFC are; sodium trimetaphosphate, sodium tripolyphosphate, epichlorophydrin, phosphoryl chloride, glyoxal, and ammonium zirconium (IV) carbonate.
In recent years a lot of effort has been devoted to develop an alternative viscosifier for guar gum because: there is a shortage in guar supply as the stimulation activity is rapidly growing; guar has a certain temperature limitations; and guar leave residues in the formation even after chemical or enzymatic treatments that are used in order to remove guar gum.
There is therefore a need for an alternative viscosifier which does not suffer from the disadvantages of guar.
According to the present invention there is provided a viscosifier for oil well fluids, said viscosifier comprising a cross-linked micro- or nano-fibrillated cellulose (MFC).
The MFC may be selected from modified MFC such as TEMPO mediated MFC, Carboxymethylated MFC and cationic MFC, Enzymatic assisted MFC, and mechanically produced MFC.
According to an embodiment the cross-linking is physical cross-linking and may be formed by a metal cation or metal complex. The metal cation or metal complex may optionally be selected from the groups consisting of aluminum sulfate (Al2(SO4)3), aluminum chloride (AlCl3), zirconium chloride (ZrCl4), chitosan, hyberbranched polymers such as polyesteramide such as Hybrane® 113, polyethyleneimine (PEI), boric acid, borax and borate salts, boron minerals (such as Ulexite (NaCaB5O6(OH)6.5(H2O)) and Colemanite (CaB3O4(OH)3.H2O)), organo-borate complexes (such as 4,4′-biphenyldiboronic acid), organometallic compounds containing Zr, Ti or Hf ions such as Tyzor® 212 and Tyzor® 215. Preferred cross-linking agents are Tyzor® 212 and Tyzor® 215.
According to another embodiment the cross-linking is chemical cross-linking. The cross-linking agent may be selected from formaldehyde, difunctional aldehydes such as glutaraldehyde, dichloroacetic acid, polyepoxides, urea, sodium trimetaphosphate, sodium tripolyphosphate, epichlorophydrin, phosphoryl chloride, glyoxal (OCHCHO), and ammonium zirconium (IV) carbonate.
The MFC may have an average diameter in the range 5-100 nm, for example in the range 5-70 nm, or in the range 10-50 nm. It may also have a length in the range 1-100 μm, for example 1-70 μm, or in the range 1-50 μm.
The invention also extends to an oil-well fluid comprising a dispersion including a viscosifier as set out above.
The advantages of using such a viscosifier in an oil well fluid will become clear from the examples that follow, but they include (i) using a smaller amount of polymer (fiber) in oil well fluids such as fracturing fluids; (ii) minimizing the formation damage due to the low amount of fiber used; (iii) simplifying and reducing the cost of the cleanup operation by using less chemicals; and (iv) improving the stability of the MFC gel (three dimensional network) toward the heat and contamination environment to which they are exposed which helps to guarantee a successful job performance.
The MFC is optionally in the form of an aqueous dispersion and the MFC may be present in an amount 1-50 g/l, or in an amount of 1-30 g/l, or in an amount 5-15 g/l. The MFC can also be in form of a non-aqueous fluid such as petroleum distillate or any types of glycols such as ethylene glycol. The concentration of MFC in such non-aqueous fluid can be in an amount of 1-800 g/l or in an amount of 100-600 g/l, or in an amount of 300-500 g/l.
In an embodiment the oil-well fluid additionally comprises a proppant and the concentration of the cross-linked MFC in the fluid is from 0.1-2.5 wt %. The proppant may be any suitable proppant, for example sand or a ceramic material.
The MFC materials used in the examples below were produced in the laboratory as described in the literature as follows.
TEMPO mediated MFC (TEMPO-MFC) was produced according to the publication of Saito et al. (Saito, T. Nishiyama, Y. Putaux, J. L. Vignon M. and Isogai. A. (2006). Biomacromolecules, 7(6): 1687-1691).
Enzymatic assisted MFC (EN-MFC) was produced according to the publication of Henriksson et al, European polymer journal (2007), 43: 3434-3441 (An environmentally friendly method for enzyme-assisted preparation of microfibrillated cellulose (MFC) nanofibers) and M. Pääkkö et al. Biomacromolecules, 2007, 8 (6), pp 1934-1941, Enzymatic Hydrolysis Combined with Mechanical Shearing and High-Pressure Homogenization for Nanoscale Cellulose Fibrils and Strong Gels.
Mechanically produced MFC (ME-MFC) was produced as described by Turbak A, et al. (1983) “Microfibrillated cellulose: a new cellulose product: properties, uses, and commercial potential”. J Appl Polym Sci Appl Polym Symp 37:815-827. ME-MFC can also be produced by one of the following methods: homogenization, microfluidization, microgrinding, and cryocrushing. Further information about these methods can be found in paper of Spence et al. in Cellulose (2011) 18:1097-1111, “A comparative study of energy consumption and physical properties of microfibrillated cellulose produced by different processing methods”.
Carboxymethylated MFC (CM-MFC) was produced according to the method set out in “The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes” Wåagberg L, Decher G, Norgen M, Lindström T, Ankerfors M, Axnäs K Langmuir (2008) 24(3), 784-795.
The rheological properties of the various fibrillated cellulose with crosslinking agents were investigated in the laboratory in fresh water, sea water, and in brine solutions at different pH level, and at different temperatures from room temperature up to 175° C.
The equipment used to measure the various properties included a mass balance, a constant speed mixer up to 12000 rpm, a pH meter, a Fann 35 rheometer, a Physica Rheometer MCR—Anton Paar with Couette geometry CC27, and a heat aging oven (up to 260° C. at pressure of 100-1000 psi).
As mentioned above microfibrillated cellulose can be produced with one of the following methods and the resulting MFC can have slightly different properties.
Mechanically produced MFC: just mechanical shearing is used for the defibrillation. The surface charge of the fibril is quite small and similar to the original fiber.
Chemically assisted process; chemicals such as TEMPO are used to lower the energy consumption and make the defibrillation easier when compared to the pure mechanical method. Such chemical treatments introduce a negative charge on the surface of the fibril which in turn might affect the crosslinking reaction.
Enzymatic assisted process; enzymes such as cellulase are used to shorten the length of the fiber and make it easier to defibrillate. The surface charge is similar to the original fiber but might change slightly.
A combination of some or all of the above methods is also possible and may be beneficial in certain circumstances. Also crosslinked MFC can be used in viscosifying oil well fluids solely or combined with any commercially available viscosifiers such as guar gum, modified guar gum, starch and starch derivatives, cellulose and cellulose derivatives, xanthen gum, synthetic copolymers such as polyacrylamide and its derivatives, acrylates and its derivatives, viscoelastic surfactant or any clay minerals such as bentonite, sepiolite or attapulgite.
The concentration of a well-defibrillated MFC aqueous dispersion is normally below 50 g/l due to the high viscosity of the dispersion. In the examples below dispersions with concentrations of 10-30 g/l were diluted with distilled water and mixed in a Warring blender before adding the crosslinking agent. The pH of the dispersion was adjusted sometime before or after the addition of the crosslinking agent. The viscosity of the dispersion with and without crosslinker was measured at room and elevated temperatures. In some examples a salt such as potassium chloride (KCl) was added to the dispersions since it can be a main component in the fracturing fluid to minimize the shale hydration.